Nitrided article of ferrous character containing a case-toughening element combined with a balanced case-forming element



' quenching.

Patented Feb. 7, '1933 UNITED STATES PATENT" OFFICE HERBERT JAMES FRENCH, OF ELIZABETH NEW J'EItSEY, AS SIGNOR TO THE INTER- NATIONAL NICKEL COMPANY, INC OF NEW YORK, N. Y., A CORPORATION OF DELA- WARE NITBIIOED ARTICLE OF FEBROUS CHARACTER CONTAINING A CASE-TOUGHENING ELEMENT COMBINED WITH A BALANCED CASE-FORMING ELEMENT No Drawing. Original application filed June 12, 1931, Serial No. 544,044. Divided and this application filed September 6, 1932. Serial No. 631,894.

This invention relates to improved nitrided articles of ferrous character containing a case-toughening element combined with a balanced case-forming element and to a process of manufacturing such improved nitrided articles, and more particularly to an improved nitrided article constituted of steel or a ferrous alloy and containing'a casetoughening element combined with a balanced case-forming element and to a process of producing such nitrided article. i

It is well known that relatively soft steels may be provided with a hard surface by case hardenin them. Heretofore, cases have been pro uced by carburizing the surfaces of the articles and then hardening them by Articles hardened in this man ner were subject to Warpage or distortion which rendered some of such articles unfit for use or necessitated finishing them mechani-v cally after treatment. Moreover, articles which were case hardened with carbon, could only be made with hardness up to about 650 to 7 50 on the Brinell hardness scale.

For roducing greater surface hardnesses in stee s, nitridmg waspro osed. As it is 1 I nickel not only has a retarding action on well known, the nitriding (nitride hardening or nitrogen hardening) processes are generally more expensive than the conventional case hardening (carburizin and hardening) processes. Steels of specia composition are required (alloy steels) and in addition the nitrided articles lacked reliability and reproducibility. Moreover, when cases of very high hardness were produced they were relatively brittle and often they cracked in use under comparatively low pressure or shocks in machinery parts. Although the art was struggling to provide improvements in nitrided steels, which would better meet the 5 demands of commercial, industrial and practical use, such improved nitrided steels have not been provided.

In the United States Patents #1,487,554 and #1,649,398 to Fry, a process of nitriding steels and an article produced thereby are set forth which describe that aluminum is an essential element in the steel to be nitrided. The Fry process essentially involves the adlimitations and disadvantages of the Fr process and product and states that Frys product is deficient in toughness and cohesiveness and that molybdenum is necessary in steel for the production of a satisfactory commercial nitrided steel. In addition, the Fry patent contains other defects. For example, Fry states that by the process de scribed. in his patents (#1,487,554 and #1,649,398) marginal layers of an extraordinarilyhigh hardness are acquired in steel alloys which contain O.5-2.0% aluminum and either separately or in any desired combination 0.5 to 4.0% silicon, manganese. nickel, chromium, molybdenum, tungsten, vanadium, titaniumor zirconium and. up to 0.6% carbon. It is now known that aluminum is not essential to the production of high hardness in the marginal layers of steel when treated by the ordinary nitriding processes such as heating in ammonia.

Other investigators have published that 4 The aforesaid effects of nickel have been investigated by Guillet as shown in his article in Genie Civil, vol; 91, pp. 38, 60, 86, 472 (1927), by Billy as shown in his articles in Krup sche onatschafte, vol. 4, p.- 137 (1923 and in Stahl and Eisen 43, 127 (1923) and Sergeson as shown in his article in the Transactions of the American Society for Steel Treating, vol. 16, p. 145 (1929) etc.

It has now been discovered that certain metals,. for. instance, nickel, molybdenum, chromium, vanadium, heretofore considered as equivalents of each other are not equivalent and interchangeable but have different and sometimes directly opposite characteristics with respect to the nitriding processes for nitriding steels and the products thereof; that some of the metallic elements heretofore considered necessary in steels for nitriding are unnecessaryfthat a substantial im-' cases of nitrided steels may be effected; that an increased nitrogen diffusion in the article may be obtained; that the highly important hardness-toughness relations and the depth of the case produced by nitriding may be controlled; and that an improved, stable, reliable and reproducible nitriding steel can be produced by incorporating a case-forming element or several of such elements conjointly with a case-toughening element or several such elements in certain relation to each other manner, such for example as .by heating in" an atmosphere of ammonia at suitable temperatures for suitable periods of time.

By case-toughening element, is meant such an element that causes an improvement in the toughness of the case. The term toughness as used in this specification represents improved ductility combined with improved strength and sometimes also combined with improved shock resistance.

According to the present discovery, the case-forming element may be an element such as chromium, aluminum, or some other element which is capable of causing in a steel or ferrous alloy, a relatively high rate of absorption of nitrogen and a relatively low rate of nitrogen diffusion, and the casetoughening element may be nickel, molybdenum or some other element which is capable of impartin to the case on a nitrided steel or ferrous al 0y, an increased ductility combined with an increased strength. The

case-toughening element or elements are addment or elements so as not only to counteract embrittling and weakening effects of the caseforming element or elements in the case on the article but also to improve the structure and other physical properties of the case formed on the article. The additions of casetoughening and case forming elements may be so controlled as to improve, at the same time, the properties of the core of the article. The elements mentioned hereinabove have as a dominant characteristic either caseforming properties or case-toughening prop erties. In some instances elements may have approximately equal case-forming and casetoughening properties. For instance, vanadium dependin upon its conditions of application inclu ing percentages present exhibits both case-forming and case-toughening properties. In percentages around 0.5%

it shows marked case-forming properties.

Generally speaking, the present invention contemplates increasing the percentage of the case-toughening element or elements as the percentage of the case-forming element or elements is increased in a steel or ferrous alloy so as to counteract certain detrimental tendencies of the case-forming element or elements as well as to efl'ect an improvement in the structure and properties of the case on the article. The percentage of the caseforming .element to be added will depend upon the properties desired, such as the strength, hardness, wear-resistance, etc., and upon the individual characteristics of the element and upon the composition of the steel or ferrous alloy to be nitrided. The percentage of the case-toughening element to be added depends upon its individual characteristics, the case-forming element and the percentage thereof added to the metals or alloys under treatment and the composition of the steel or ferrous alloy to be nitrided and upon the properties desired in the final product.

In the practice of the invention, it is preferred to use nickel and molybdenum conjointly as the case-toughening elements because not only tough cases can be provided on the article but also improved core properties not attainable by the use of either of these elements separately can be obtained. It is preferred to have the nickel addition equal or exceed one part of nickel to one part of molybdenum (by weight) but the benefits obtained from the nickel and molybdenum as case-toughening elements are not restricted to these preferred proportions as is shown by the examples hereinafter described.

Generally speaking, it is preferred to use such a percentage of case-toughening element that the ratio of the percentage of casetoughening element to the percentage of the caseformin'g element is about 0.8:1 when improved case-toughness is desired Without an appreciable decrease in maximum hardness. Ratios of not less than about 0.8: 1.0 and not more than about 5: 1 should be used, the ratio depending upon the characteristics of the case-forming elements added and upon the properties desired in the nitrided steels. The higher ratios of case-toughening element to case-forming element are employed in those instances where the maximum case hardness desired becomes lower. In Dractice, it has been found that a 'ratio of 3:1 gives satisfactory and useful results. l-Vhen the prior art ratios of less than about 0.8:1 are employed, practically no case-toughening is effected and the case on the article is defective and brittle. On the other hand, when the prior art ratios of higher than about 5: 1 are employed the case on the article does not possess s'uflicient hardness and is termed soft in relation to ordinary nitrided cases.

For a better understanding of the invention, the following specific and illustrative examples are given:

Example N o. 1

In accordance with the invention a steel, for instance, has incorporated therein about 2% (by weight) of caseforming elements, which may be about 1% (by weight) of aluminum and about 1% (by Wei ht) of chromium, and about 1%% (by weight) of casetoughening elements, which may be about 1 (by Weight) of nickel and about 4% (by weight) of molybdenum. The composition of the steel may be as follows (by weight) (case-tough The case-forming elements and the casetoughening elements may be incorporated in the steel in any convenient manner. It is preferred, however, to add the aforesaid elements to the steel while the latter is in a molten condition. After the addition to and incorporation in the steel of said elements, the steel may be cast into ingots or the like and articles made therefrom. The articles may then be subjected to nitriding in any approved or suitable manner, as one skilled in the art will readily understand. Preferably, however, the articles are heated in ammonia at 900--1000 F. for a customary period of time. Various temperatures, periods of time, etc., may be used and the following table gives some of the customary temperatures, periods and cycles used in nitriding:

Table 1 Temperature Period oi time For 1 to 100 hours.

For 1 to 100 hours from 1 hour up.

From 1 to 48 hours for 1 hour up.

For 1 to 100 hours.

Nitride at a temperature between 000 and 1000 F. Nitride as above followed by 1200 F Nitride at about 1200 F. followed by one temrature between 900 to 1000 F.

Nitride at a temperature between 900 and 1000 F. Gradually increase temperature to 1150 F. with increase in heating rate as temperature rises. End at 1150 F.

properties are tabulated of articles made from a similar steel substantially devoid of the combination of the case-forming elements and the case-toughening elements.

Table 2 x 23 333} Case toughnew ness meas- Steels with small amounts oi 0,. monotrom ured by Steel Si, Mn and customary impuri- Brine (at load in kg ties, and or new on Brmell surface of 23 3 article) A No alloy additions 250 No meticase to crack. B Daily tgsfd'orming elements (1% A1, .920 275 r C Case-iorming elements with insui- 890 300 ficient case toughening elements (1% A1, 1% Cr 54% Mo). D Balanced caselorming elements 800 555 and ea'setoughenlng elements or. 2% o, was

The foregoing samples were nitrided for about 48 hours atabout 975 F. with about 25/30% dissociation of ammonia.

The addition of the case-toughening elements (l of nickel combined with 4% molybdenum) together with 2% of caseforming elements (1% aluminum and 1% chromium) has resulted in greatly improved case-toughness without an appreciable change in maximum case hardness as compared to a steel containing only case-forming elements (steel B, Table 2) or case-forming elements with insuiiicient quantities of case-toughenin elements (steel C, Table 2).

he steel D of Table 2 containing nickel has a higher core hardness and strength, as normalized, or when quenched and then tempered at 1200 F. to 1300 F. than a corresponding steel without the nickel treated in a-corresponding manner and will give better support to the case when subjected to heavy pressures; In addition, the case of the nickelcontaining steel is far superior to that of the corresponding steel without nickel. It has high hardness (compare steels C and D, Table 2) and, at the same time, it is much tougher and when deformed will not break as readily as the case of the corresponding steel without nickel.

The nickel-containing steel is therefore especially well adapted to applications subjected to heavy loads such, forexample, as gears in automotive construction, shafts in machinery, etc.

amounts appreciably above about 2 per cent are added to steels containing approximately 1% of eachof the case-forming elements aluminum and chromium, a substantial increase in case toughness is obtained when the steel is nitrided in ausual manner, viz,-

about 48 hours at about 975 F. with about Y 25/30% dissociation of ammonia.

Each of the foregoing results is the average v of two tests.

Table.

- Table 5.Bend test results on steels E, F and G.

(Specimens g inchx tnclt 7 inches-teated Case tough on 1; inch span) Steels with small amounts of C Si ness meas- Mn and customary impurities 5352 ured by Steel toggthggvith approiililmalteiy 1 tg ness'momy ioarliain kg Load hubs nefilection I eac senseorm cemen 0]] H110 h under, andg tron-B baiito steel 3.5.3.?

crackcase I Case me E Cnwtoughening elements as 01- 8 0 0 E (No added nickel) 2835 .044 lows: practically none. F (3.5% added nickel) 4180 i060 F 3.53%N and 54% Mo 7 960 G added nickel) 4676 .067 Q 5.01% and 54% Mo 720 1140 The foregoing samples were nitrided for The foregoing samples were n1tr1ded for about 48 hours at about 975 F. with about I- I u I I about,48 hrs. at about 915 F. with about 25/30% dlssoclatlon of ammoma.

25/30% dissociation of ammonia.

he increase in toughness produced by the relatively large increases of the case-toughening element nickel is accompanied by a decrease in the maximum case hardness (observed at or near the surface of the articles).

the amount of the decrease in the hardness.

and the increase in the case toughness being controllable by the amounts of the nickel added.

The fore oing example clearly demonstrates that t e present invention provides the tempering subsequent to quenching, which is effective in carbon case hardened steels, is not, as one skilled in the art knows, available in the case on nitrided steels.

The marginal or surface layers of my ime proved nitrided articles will not only better resist cracking of the case but will withstand higher'loads than heretofore and may be de;.- formed further than hitherto without cracking. These features may be shown and demonstrated-by the following table showing results of torsion and bend tests obtained upon steels E, F and G of Table 3.

Tablet-Torsion test results on steels E and G (apecimem 64. inch diameter-8 inches long) Angle at Torque to n I 5 steel flfi f ia tii g degree g per inch E (No added nickel) 5130 2. 5 G (5% added nickel) 09w .0

Each of the foregoing results isthe average of two tests.

Many tests in which the nitrided surfaces have been removed to various depths before testing have indicated that those torsion test results and bend test results are typical of the beneficial influences throughout the hard marginal layers (case) of the nitrided articles. l

Ewample N o. 3

The invention may be illustrated further by adding to low carbon steel containin small amounts of silicon, manganese and t e cus tomary impurities, case-forming elements, comprising about 1 er cent of chromium and about 0.5 per cent 0 vanadium, together with about 1.6 per cent of the case-toughenin elementnickel. The steel may have the fo lowing composition C about 0.14% (b weight).

Mn about 0.62% by weight).

Si about 0.18% (by weight).

P about 0.02% by wei ht).

S about,0.03% Ew'ei %1t).

Cr about 1.16% Sly weight).

I V about 0.55% y weight). Ni about 1.63% (byweigbt). Fe about balance (by weight).

Comparison of this steel with the corresponding one in which the case-forming ele-.

'ments have not been balanced against. the

case-toughening elements shows the marked superiority of the former. The. addition of the nickel results in an appreciable increase in the case toughness without appreciable loss in the maximum case hardness.

The torsion test results which onstrate the ability to control the hardnesstoughnessrelations of the marginal laye s of nitrided articles throu h nickel additions adjusted to the 'caseorming elements follow dem-' Table 6 Torque in inchlbs. to crack case after removing the speclfied depth of case by grinding subse uent to mt: ding An e, degree per inc to crack case after removing the specified depth of case subsequent Steel 0 to nltrldmg None 0.002 0.008 None 0.002 0.008

None 6090 l. 2 2. 1 2. 1 1. 63 6990 2. 0 6. 8 5. 9

The steels listed in Table 6 were nitrided about 48 hours at about 975 F. with about 25/30% dissociation 'of ammonia. The test specimens were inch diameter by 8 inches long. All results are from tests in duplicate.

Emample No. .4

One aspect of the invention may be further illustrated by the following: If, to a steel containing small amounts of carbon and the customary impurities, there is added about 1 per cent. of a case-forming element, such as chromium, and insuflicient amounts of case-toughening elements, say only per cent of molybdenum, the resultant hardnesstoughness relations of the marginal layers of nitrided articles made therefrom will not be as good as those which can be secured by balanced additions made according to this invention. In this instance, the addition of about 1% per cent nickel combined with the aforesaid alloy additions completes the proper amount of case-toughening elements to give a relatively high degree of toughness without an appreciable decrease in maximum case hardness. This illustrates one feature of the control of the hardness-toughness nelations of nitrided articles. The following table illustrates the advantages of this feature of the present invention:

25/30% dissociation of ammonia.

E wample N 0. 5

The effectiveness and usefulness of nickel as a case-toughening element in nitrided steels and articles made therefrom, which improvement constitues one aspect of this invention, may be further illustrated by comparison of the results of tests of a steel containing about 2% chromium and a steel containing about 2% chromium and about 2 of nickel, viz:

Table 8 Mfifilmdlilm itoiiighness case aiess on n g. on Steel 0 01' monotron- Brlnell ball to Brinell crack case 0 20 2. 08 None 780 2(1) P 25 2. 53 2. 61 830 525 The foregoing samples were nitrided for about 48 hours at. about 900 F. with about 25/30% dissociation of ammonia. 1

The balanced composition of the steel P of Table 8 made according to the present invention has very much higher case toughness without appreciable loss in maximum case hardness as compared to the steel 0 of Table 8 with unbalanced composition comprising case-forming elements without compensating case-toughening elements.

Example N0. 6 I

As has already been set forth, one of the important aspects of this invention is the practicability of the control of the hardnesstoughness relations of the marginal layers (cases) of nitrided steels and articles made therefrom by balancing the additions of caseforming elements and the additions of casetoughening elements. Nickel is especially effective in the group of case-toughening elements which may be used for this'purp'ose as is shown by the results of the experiments which follow. The following table shows a steel Q, with no case-toughening element, and steels S and T which embody the principles of the present invention.

Table 9 v Case tough- Steels with about 1% Al together w ness load em] with small amounts 016 S Mn ness! in kg. on

:ar n'namie as o ow ons o Brlnell and: case 1% No added case-toughening element... 806 115 3.5 added nlckel..- 045 960 'r 5.1 0 added n1cke1..-.... 640 1740 The foregoing samples were nitrided for about 48 hours at about 975 F. with about 25/30% dissociation of ammonia.

As is shown in Table 9,-by adjusting the relative proportions of the case-forming elements and case-toughening elements in the steel (in this case by varying the amounts of the'case-toughening element, nickel), it

has been practicable to control the hardnesstoughness relations of the marginal layers of the nitrided articles within relatively wide limits. With about 3.5 to 5 per cent 'added nickel, there was very marked increase in the case toughness with a corresponding decrease in the maximum case hardness. The amount of the increase in the case toughness and the and a relatively high toughness,,higher. ratios decrease in the maximum case-hardness was dependent upon the proportions of nickel added that is to say, upon the adjustment of the proportions of case-forming elements and case-tougheningelements present.

In addition to the foregoing examples, the following illustrative exam les are'embodiments of the present invention and are listed in a table for convenience. In Table the It will be observed that the present invention provides nitrided steels having controlled case-hardness and case-toughness and a process by ,which such nitrided steels may be produced. In the instances where a steel or ferrous alloyis desired with a nitrided case having relatively high hardness and a relatively good toughness, the ratio of case-toughening element to case-forming element present in the steel or ferrous alloyis about 0.8 1. .On the other hand, in the instances where a steel or ferrous alloy is desired with a nitrided case'having a relatively good hardness of case-toughening element to case-forming j; element, of the order of 'about'3: 1 and about 5' 1 are used. .In no case, however, is a. ratio 'greaterthan about 521 employed. Theds'p'e: cific ratio in any given .casewlll also depend partly upon the temperature a'ndtime ofnitriding,"ca'rbon contentof the steel or ferrous alloy, characteristics of case-toughening eleelements, etc. The essential feature that the case-toughening element must. be bal aneed or controlled with respect-t0 the case'- forming element.

By means of balancing (or controlling) the case-toughening element to the case- ;forming element as described herein, the advantages, new and/or improved results set forth in detail hereinabove may be obtained. In addition, if cases of equal depth are desired the time of nitriding may be shortened by the use of the present process from about 5% to 25% or more. Moreover, with corresponding nitriding procedures, it is possible to produce by use of the present invention ment or elements, of case-forming element orcases deeper than those obtained in the conventional nitrided steels.

In carrying the present invention into practice, various percentages (by Weight) and/or combinations of the different casetoughening elements and case-forming elements may be used. In general, steels and ferrous alloys containing the following elements and percentages may be employed to carry the present invention into practice; aluminum 0% to about 5%; chromium 0% to about 25%; nickel 0% to about 16%; molybdenum 0% to about 12%; and vanadium 0% to about 2%. The present invention may be carried into practice with steels or ferrous alloys including cast iron, malleabilized cast iron, and substantially carbon-free iron alloys.

When iron, steel or ferrous alloy is referred to in the specification and claims, it is to be understood that the usual amounts of customary elements such as silicon, manganese, sulfur, phosphorus, etc. are present. In the claimsthe ex ression an element of the chromium type is meant to refer to case-forming elements as described herein, Whereas the expression an element of the nickel type is meant to refer to case-toughening elements as described'herein.

The crack test applied to nitrided cases on steels and ferrous alloy articles refers to a test in which the nitrided case is subjected to stress as by the ball of a Brinell hardness testing machine and the case is heard to crack. In practice, the occurrence of a sharp noise or crack denotes that mechanical discontinuity, such as arupture, has developed in the nitrided case. By carrying the test slightly further, the crack can be confirmed i by theuse of a microscope, by usual examination or by a bend test or by a torsion' test.

. It will be noted that the present invention provides an improved article ofmanufacture comprising a ferrous body having a nitrided case and'containing a case-toughening element and a case-forming element in such amounts that the ratio of the former element to the. latterv element is not less than about 0.851 and not greater than-about 5:1.

jIt will also be noted that the-presentinvention provides an improved-mitrided steel or the like having relatively high hardness combined with relatively high; toughness in which the case has substantial depth without any detrimental tendency to surface scaling and in which the core or body of the steel has sufficient hardness and toughnessv to support the case even under relatively severe loads or impacts. v.

When in the claimsa nitrided case is referred to'as' having a relatively high hardness'and relatively high toughness or relatively high hardness combined with relatively high toughness,hardnesses and tough nesses of the order of those set forth in the foregoing tables are meant.

It is to be furtherobserved that the present invention provides'a new nitrided ferrous article of manufacture containing a case-toughening element and a case-forming element in certain proportions to each other. In regard to a case-toughening 5 element, molybdenum is an equivalent of nickel. When the Kenn nickel is used in the claims, it is to he nderstood that molybdenum is to be considered within the purview of said' terms.,' It is to be understood that the same "percentages of the three elements.

results are not necessarily secured by 118% course, aluminium may be used alone, especially where retention of high case hardness at high temperatures is desired. There are often" advantages in using a combination of two or more of these case-forming elements. For example, chromium and aluminium may be used with satisfactory results. I While specificreference has been made to treatment in ammonia, the described benefits resulting from the employment of the prini: ples of the present invention, processes and products described, are not limited as to the sources of the nitrogen which is absorbed in the marginal layers of the steel to form a case.

The present application is a division of my copending application, U. S. Serial No. 544,044, filed June 12, 1931.

I claim: v

1. An article of manufacture com rising a 3.0% chromium and a material per cent to about 1% of vanadium acting as case formlng elements, the ratio of said case-toughening element to said case-forming elements being not less than about 0.8:1 and not greater than about 5:1, and the case-forming ele-- ments being present to an extent of more than about 1.5% and less than about 5% and the case-toughening element being present to an extent less than about 6%.

3. An article of manufacture com rising a v ferrous body having a tough nitride case, as

nitrided, and containing about 2% to about 5% of nickel and a material percent to about 0.5% of molybdenum acting as case-toughening elements balanced with about 1% to about 2% chromium and a material per cent. to about 1% of vanadium acting as caseforming elements, the ratio of said casetoughening elernents to said case-formin elements being not less than about 0.8: 1 an not greater than about 5: 1, and the case-forming elements being present to an extent of more than about 1.5% and less than about 5% and the case-toughening elements being present to an extent less than about 6%.

In testimony whereof, I have hereunto set my hand.

HERBERT JALHES FRENCH.

as nitrided, and containing nickel actin as a case-toughening element combined wit at least one balanced. case-forming element of I the group consisting of chromium and vanadium, theratio of said case-toughening element to said case-forming element being not less than about 0.8:1 and not greater than about 5: 1, and the case-forming element being present to an extent of more than about 1.5% and less than 4% and the case-toughening element being'present to an extent less than about 6%.

2. An article of manufacture comprising a ferrous body having atough nitrided case, as nitrided, and containing about 1.5% to about 5.0% of nickel acting as a case-toughening element balanced with about 1.5% to about 

